1. Field of the Invention
The present invention relates to a sensor circuit for sensing a dielectric constant of a substance to be measured or a concentration of a specific component, and more specifically to an electrostatic capacitance sensing circuit using an oscillator whose frequency changes according to an electrode capacitance formed by a substance to be measured and sandwiched between two opposing electrode plates.
2. Description of the Prior Art
FIG. 1(a) shows a prior-art Clapp oscillator capacitance sensing circuit, which is disclosed in Japanese Published Examined (Kokoku) utility Model Appli. No. 57-7010, and FIG. 1(b) is an equivalent circuit thereof.
The electrode 1A is disposed within a vessel 3 in which a substance 2 to be measured is put so as to form an electrode capacitor 1. The substance 2 to be measure is a mixture of gasoline (the dielectric constant or the relative permittivity: .epsilon./.epsilon..sub.0 .apprxeq.2) and methanol .epsilon./.epsilon..sub.0 .apprxeq.30), by way of example. The dielectric constant .epsilon..sub.1 /.epsilon..sub.0 of this mixture changes according to the mixture ratio of gasoline to methanol. Further, the electrostatic capacitance C.sub.1 between the electrode plates 1A changes roughly in proportion to the dielectric constant .epsilon..sub.1 /.epsilon..sub.0 of the substance 2 to be measured, and therefore can be expressed as ##EQU1## where A(cm.sup.2) denotes each opposing area of the electrode plates; d(cm) denotes the distance between the two electrode plates; and .epsilon..sub.0 =8.854.times.10.sup.-14 (F/cm).
FIG. 1(c) shows the relationship between the electrostatic capacitance C.sub.1 and the concentration M (by volume %) of methanol. The electrostatic capacitance C.sub.1 formed by a substance 2 between a pair of the electrode plates 1A is referred to as electrode capacitance C.sub.1, hereinafter.
The well-known Clapp oscillator shown in FIG. 1(a) is mainly composed of a resonance circuit and a transistor 9.
The resonance circuit is composed of an inductance (a coil) 6, four fixed capacitors 4, 5, 7 and 8, and an electrode capacitor 1, as depicted in FIG. 1(a). Further three biasing resistors 10, 11 and 12 are connected to the transistor 9 to apply appropriate bias voltages to the base and the emitter of the transistor 9.
Since the electrode capacitor 1 and capacitor 4 are connected in series; the two capacitors 7 and 8 are also connected in series; and further the capacitor 5 is connected in parallel to the two series-connected capacitors 1, 4 and 7, 8, the oscillation frequency f.sub.0 of this Clapp oscillator can be expressed as ##EQU2## where C.sub.1 denotes the electrode capacitance; C.sub.4, C.sub.5, C.sub.7 and C.sub.8 denote electrostatic capacitances of the capacitors 4, 5, 7 and 8; and L.sub.6 denotes the inductance of the coil 6, as indicated in the equivalent circuit shown in FIG. 1(b).
The above expression 2 indicates that the Clapp oscillator frequency changes in inverse proportion to the electrode capacitance C.sub.1. Further, in FIG. 1(a), the emitter output signal of the transistor 9 is applied to the F-V converter 14 via a capacitor 13 so that an output voltage E(v) can be changed according to the oscillator frequency f.sub.0. Therefore, it is possible to measure the mixture ratio of gasoline and methanol on the basis of the output voltage E(v).
FIG. 2 shows a classical Hartley oscillator capacitance sensing circuit, as disclosed in Japanese Published Examined (Kokoku) Patent Appli. No. 29-8000. In this oscillator, the electrode 1A is connected in parallel to a coil 15 having a tap 15 connected to a cathode of a vacuum bulb 16 so as to form a resonance circuit composed of parallel-connected electrode capacitor 1 and the coil 15. Further, an end of the coil 15 is connected to a grid of the same vacuum tube 16. To detect the oscillator signal, another coil 17 is induction-coupled to the coil 15 so that an output signal whose frequency changes according to the electrode capacitor 1 can be outputted.
FIG. 3 shows another classical oscillator capacitance sensing circuit, as disclosed in Japanese Published Examined (Kokoku) utility Model Appli. No. 44-17838. In this oscillator, a feedback loop formed between a plate and a cathode of a vacuum bulb 18 is composed of a capacitor 21, a resistor 22, a first coil 19, a second coil 20 induction-coupled to the first coil 19, and a parallel-connected resistor and capacitor. The electrode capacitor 1 is connected in parallel to the first coil 19 so as to form a parallel resonance circuit. In this circuit, the frequency of the oscillation signal obtained through the plate of the vacuum bulb 18 changes according to the electrode capacitance C.sub.1. Further, to detect the oscillator signal, a grid of another vacuum bulb 24 is connected to the plate of the bulb 18 via a capacitor 21 and a resistor 23 so that an output signal can be outputted after having been amplified by the bulb 24.
In the prior-art oscillator capacitance sensing circuits, however, there exists a problem in that the sensing reliability is poor, because a bias voltage of an active element is directly applied to the electrode 1A within which a substance to be measured is disposed.
In more detail, in FIG. 1(a) a bias voltage applied to the base of the transistor 9 is applied to the electrode 1A via the capacitor 4; in FIG. 2 a bias voltage applied to the grid of the bulb 16 is directly applied to the electrode 1A; and in FIG. 3, a bias voltage applied to the grid of the bulb 24 is applied to the electrode 1A. Therefore, there exists a problem in that the electrode plates are subjected to electrolytic corrosion due to a relatively high dc bias voltage applied to the active element (i;e. transistor, bulb, etc.) which constitutes an oscillator circuit.
This is because when a relatively high dc voltage is applied to the electrode 1A, since ions are dissolved into the substance 2 to be measured, the substance 2 changes to an electrolytic solution, so that an electromotive force is generated between the two electrode plates to cause electrolytic corrosion.
Here, in FIG. 1(a), it should be noted that although a capacitance 4 is connected in series between the electrode capacitor 1 and the coil 6, since the capacitor 5 is connected in parallel to the series-connected two capacitors 1 and 4, a high bias voltage is still applied to the electrode 1A.